1. Field of the Invention
The present invention relates to a device for sensing deterioration of a light-emitting diode such as a laser diode (LD), and more particularly, to a device for sensing deterioration of an output power of the LD.
2. Description of the Related Art
In an LD scanning device which utilizes an LD as a light source, a light intensity of the LD should be constant to generate a uniform or stable image pattern, and therefore, it is necessary that the light-emitting output of the LD is checked and, when the LD has deteriorated causing the light-emitting output to decrease, a proper treatment is applied to the LD scanning device. The LD has characteristics in which a light intensity increases with a linear relationship, as an electric current supplied to the LD increases past a threshold value of current at which the LD begins to emit a laser beam, and when the LD has deteriorated, the characteristics are changed, so that the inclination of the linear relationship becomes less, or that the threshold value increases. Thus, when an electric current is supplied to an LD which has deteriorated, the light-emitting output of the deteriorated LD is reduced in comparison with that when the same value of current is supplied to a normal LD. A factor in the deterioration could be a secular change of the characteristics at the semiconductor junction of the LD, or an electrostatic discharge failure at the semiconductor of the LD.
In an LD circuit having an APC circuit, in which the light-emitting output of the LD is sensed to feed-back control of the supplied current to the LD, based on the sensed light-emitting output so that the light-emitting output remains constant, if the LD has deteriorated, the supplied current may be abnormally increased, which may cause excess current in the LD, or an extraordinary increase in electric power consumption.
To prevent the problems caused by the deterioration of the LD as described above, a device that senses the deterioration of the LD has been proposed. In this device, a resistor is connected in series with a current supplying line connected to a LD drive circuit which supplies electric current to the LD through the current supplying line, and based on an electric voltage generated in the resistor, the deterioration is checked.
In the device, however, since the resistor is connected in series with the current supplying line, a voltage drop occurs because of the resistor, and the light-emitting output of the LD or an light-emitting efficiency may be decreased. Further, because the resister also functions as a damping resister, a responsiveness of the LD becomes low. Namely, especially in a laser scanning device in which a light-emitting control of the LD is carried out at a high speed, for example, in a laser scanning device in which the current supplying line is turned ON and OFF based on an image pattern signal so that a light-emitting operation of the LD is pulse-width controlled, the responsiveness of the LID relative to the ON-OFF controlled current is reduced. This may cause the accuracy of the image pattern to be decreased.
Therefore, an object of the invention is to provide a deterioration sensing device by which a deterioration of a light-emitting diode can be properly sensed without lowering the responsiveness of the light-emitting diode.
According to the present invention, there is provided a deterioration sensing device comprising a light-emitting diode, a light-emitting diode driver, a light-receiving diode, a data output processor, a light-emitting control circuit and a deterioration sensing circuit.
The light-emitting diode driver that supplies an electric power to the light-emitting diode so that the light-emitting diode performs a light-emitting operation to radiate a light beam. The light-receiving diode senses the light beam. The data output processor outputs light-emitting data in accordance with the light beam sensed by the light-receiving diode. The light-emitting control circuit controls the light-emitting diode driver based on the light-emitting data, so that the light-emitting diode performs the light-emitting operation to radiate the light beam at a constant intensity. The deterioration sensing circuit compares the light-emitting data with a standard data, so that the deterioration sensing circuit outputs a deterioration signal when a relationship between the light-emitting data and the standard data satisfies a predetermined condition.
Further, according to the present invention, there is provided a deterioration sensing device comprising a light-emitting diode driver, a light-receiving diode, a data output processor, a light-emitting control circuit and a deterioration sensing circuit.
The light-emitting diode driver supplies an electric power to the light-emitting diode so that the light-emitting diode radiates a light beam. The light-receiving diode senses the light beam. The data output processor outputs light-emitting data in accordance with the light beam sensed by the light-receiving diode. The light-emitting control circuit controls the light-emitting diode driver based on the light-emitting data, so that the light beam has a constant intensity. The deterioration sensing circuit compares the light-emitting data with a standard data, so that the deterioration sensing circuit outputs a deterioration signal in accordance with a difference between the light-emitting data and the standard data.